The present invention relates to a method and an apparatus for measuring the concentration of suspended substances in a flowing medium by illuminating the medium with light from a light source to detect light reflections which are indicative of particle concentration. The reflected light is detected in at least two detectors; the other for measuring directly reflected light and one for measuring multiply reflected light. Directly reflected light [refers to] [the] light received from an area of the medium that is directly illuminated by the light source and multiply reflected light is light received from an area of the medium that is rather not directly illuminated, but is illuminated by light that has been scattered from the direct beam. The apparatus is preferably intended for use in so-called consistency measurements, i.e. measurements of the content of fibres in fibre suspensions, such as is found in the pulp and paper industry. The consistency is defined as the weight of fibres per volumetric unit. It is therefore a definition of concentration of suspended solids, and will be so referred to, since the present invention may be used to measure any material that scatters light in the flowing medium.
A number of different methods have been developed for measuring the concentration of fibres in fibre suspensions. The primary methods are based on mechanical principles. In particular, a shear force principle particularly has come into use during the last 50 years. In shear force measurements, the network strength of the fibre suspension is measured first with the aid of a blade, a rotating sensoror the like. The relationship between the concentration and the network strength is then calculated by a special calibration method. More recently, however, optical methods have emerged. There are several reasons for of optically based methods. Firstly, the mechanical meters of the shear force or viscosity type normally require a great deal of maintenance. Secondly, the accuracy, particularly with stationary measurement, is relatively poor with mechanical apparatus. Thirdly, great technical advances were made in the optical and electronic fields, particularly in the latter, which have facilitated and reduced the cost of meters based on optical principles. Also, an accuracy may be obtained with electro-optical techniques which is at least as good as, and in some cases better than, that obtained with corresponding mechanical apparatus.
The disadvantage of electro-optical methods has, however, been the difficulty of measuring higher concentrations. One has therefore normally been limited to measurements in the range under 10 g/liter (1%). This is due to the fact that the primary choice has been to use transmission measurement, i.e. light transmitted through the fibre suspension is detected, rather than light reflected from the suspension. Since the intensity of the light passing through the suspension declines according to an exponential function, the signal that can be generated from the light is close to the signal noise threshold for even relatively low concentrations. Devices based on polarized transmitted, light also suffer from the same limitation.